Signaling system



Jan. 4, 1944. H. E. VAUGHAN 2,338,216

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Filed July 1, 1942 v 11 Sheets-Sheet l1 INVENTOR By E lflUGHA/V 'JRLQM nrrokm'r Patented Jan. 4, 1944 SIGNALING SYSTEM Henry E. Vaughan, Valley Stream, N. Y., asslgnor to Bell Telephone Laboratories, Incorporated. New York, N. Y., a corporation of New York Application July 1, 1942, Serial No. 449,273

7 Claims.

This invention relates to signaling systems and particularly to telephone systems in which voice frequency signaling currents are employed.

Objects of the invention are the provision of more reliable signal transmitting and receiving means in systems employing voice frequency signaling and the prevention of false operation in response to voice currents or other interfering currents.

This invention is a signaling system comprising means for transmitting signals comprising current of a first frequency followed by current of a second frequency or for transmitting signals comprising current of the second frequency followed by current of the first frequency, and means for responding to such signals without responding to voice currents or other interfering currents. Similar transmitting and receiving means are connected to both ends of a toll telephone line, different frequencies being used for signaling in each direction thereover. Means are provided for opening the talking connection over the associated toll telephone line whenever signals are being transmitted or received thereover, the connection opening means being nonresponsive to voice currents or other interfering currents having a signaling frequency component.

A feature of the invention is the provision of means in a signaling system of the aforementionedcharacter for holding open the talking connection between the toll line and the associated toll route selector, through which an incoming call is extended, while line-busy or all-trunksbusy flashing signals are being transmitted over the toll line, thereby to prevent the alternate opening and closing of a direct current supervisory circuit either alone or in combination with a superimposed busy tone from interfering with the operation of the associated signal receiver in response to a disconnect signal which is being received from the calling toll office at the same time that the busy signal is being transmitted to the calling ofilce.

A clear and complete understanding of the invention will be facilitated by considering a system embodying the invention and its features, one such system being represented schematically in the drawings which form a part of this specification. The invention is not limited in its application to the particular system and circuit arrangements shown in the drawings but is applicable generally to any voice frequency signaling system.

Referring to the drawings:

Figs. 1 to 5, inclusive, represent a first toll ofhce comprising an operator's position, an outgoing trunk circuit OTI, an incoming trunk circuit ITI a two-way trunk circuit TWTI, a toll line circuit TLCI, a signal transmitting circuit STI and a signal receiving circuit SRI;

Figs. 6 to 10, inclusive, represent another toll ofilce comprising toll line circuit TLC2, signal transmitting circuit 8T2, signal receiving circuit SR2, two -way trunk circuit TWTZ, outgoing trunk circuit 0T2, toll route selector T52, and incoming trunk circuit 1T2;

Fig. 11 shows a combined intertoll and toll transmission selector CS in the second toll office and a toll connector TC in a local step-by-step dial office; and

Fig. 12 shows the relative position of Figs. 1 to 11, inclusive, to form an operative arrangement.

Reference may be had to the patent to R. E. King et al., No. 2,209,777, granted July 30, 1940, for a detailed disclosure of the cord and operator's position circuits provided in each of the two toll offlces for interconnecting calling and called lines and trunks and controlling the completion of toll calls. The outgoing trunk circuits OT! and GT2, incoming trunk circuits ITI and 1T2, the two-way trunk circuits 'I'WTI and TWT2, and the toll route selector circuit TS2 are all similar to corresponding circuits in the aforementioned King et al. patent. The intertoll and toll transmission selector CS, shown in part in Fig. 11, is similar to that disclosed in the application of P. W. Wadsworth, Serial No. 384,238, filed March 20, 1941. The toll connector TC, shown in part in Fig. 11, is similar to the connector TC disclosed in the patent to E. D. Butz, No. 2,021,286, granted November 19, 1935.

The toll line circuits TLCI and TLCZ, and the associated signal transmitting and receiving circuits STI and 5T2, and SRI and SR2, are arranged for voice frequency signaling over the associated toll lines; whereas in the aforementioned King et a1. patent, composite signaling circuits are associated with the toll lines. The toll line circuit TLCI comprises a repeating coil 40!], attenuation pads PM and P42, pad control relays M0 and ll l, a directionally selective coil 420, a cut-off relay 425, and a repeater termination consisting of condenser I22 and resistor 424 in series. The directionally selective coil 420 permits signaling current incoming over toll line T12 to pass to the signal receiver SRI and permits signaling current from transmitter STI to pass out over line Ti 2; but coil 420 prevents the passing of any substantial portion of the signaling current outgoing from transmitter STI into the signal receiver SRI. The normally operated cut-off relay 425 is released to open the talking connection between toll line TH and the trunk circuits OT! and ITI whenever signals are being sent out by transmitter STI or received by the signal receiver SRI. The toll line circuit TLC2 is similar to the circuit TLCI, the reference characters of like elements in these circuits having the same tens and units digits.

The signal transmitter STI comprises relays 43l, 433 and 435 connected in series for control by seizure, selective and supervisory direct current signals transmitted by signal relay I40 of the outgoing trunk circuit OTl. Relays 432, 434,,

438, MI, 444 and 445 are controlled by relays 43l, 433 and 435 to transmit signals consistin of current of one of two signaling frequencies either followed or preceded by current of the other frequency for required intervals of time. Each of relays 435, 438 and 444 is a polarized relay having a normally energized biasing winding for normally holding the contacts in the position shown in the drawings. Each of relays 44l and 445 has a normally energized operating winding which normally maintains the contacts in the position shown in the drawings; and has a normally energized biasing winding which is effective to move the contacts to their alternate position when the circuit through the operating winding is opened. The tone supply circuit consists of two oscillators of known design, one comprising the vacuum tube 410 for generating oscillations of one of the two signaling frequencies and the other comprising the vacuum tube 480 for generating oscillations of the other of the signaling frequencies employed for transmitt ng signals over toll line Tl2 to the second toll ofhce. The signal transmitter circuit ST2 is similar to the transmitter circuit STI, the reference characters of like elements in these circuits having the same tens and units digits.

The two frequencies used for transmitting signals over toll line Tl2 in one direction are different from the two frequencies used for transmitting signals over toll line Tl2 in the other direction; for instance, the frequencies 680 cycles per second and 1615 cycles per second may be used in one direction and the frequencies 850 cycles per second and 1870 cycles per second may be used in the other direction.

The signal receiver circuit SRI comprises an input transformer |0, an amplifier comprising vacuum tube 5l5, transformer 520, a peak limiting amplifier comprising vacuum tube 525. tuned networks 530 and 540, rectifiers 533 and 543, relay 534 for response to current of one of the two frequencies used for transmitting signals over toll line Tl2 from the tandem office, and relay 544 for response to current of the other of these two frequencies. Although the directionally selective coil 420 prevents the pass ng of a large portion of signaling currents from transmitter STI to receiver SRI, further protection against response to outgoing signaling currents is provided by a suppression network tuned to the outgoing signal ng frequencies connected across the left windings of transformer 5l0. This network comprises condenser 505 and inductor 508 in series with a filter consisting of inductor 501 and condenser 508 in parallel. In order to secure a signal response, it is necessary that the s gnaling current of each frequency comprising a voice frequency signal incoming to the signal receiver be somewhat stronger than the sum of all other currents present at the time, and that current of the second of the two frequencies constituting a signal follow the current of the first frequency within a predetermined interval of time. Only if both of these conditions exist, will a signal operation be effected; and for this reason voice currents or other interfering currents will not effect a signal operation. A high resistance element 52l, shunted by a small condenser 522, is connected in series with the grid of tube 525; and a non-linear impedance element 523, for instance,

a piece of silicon carbide, is connected across the right windings of transformer 520 to limit the voltage applied to the grid of tube 525 and thereby to limit the current in the plate circuit of tube 525. If the desired limiting action is obtainable by the use of the resistor 52l and condenser 522 alone, the element 523 may be omitted; or if the desired limiting action is obtainable by the use of the non-linear impedance element alone, the resistor 52! and condenser 522 may be omitted. Although the plate current of tube 525 never exceeds a definite limiting value, the relative proportion of the various energy components in the output is at all times the same as that of these components in the input. In the plate circuit of tube 525, the network 530 offers substantially infinite impedance to current of one of the two signaling frequencies and the network 540 offers substantially infinite impedance to current of the other of the two frequencies so that relay 534 is energized only by the part of the current which is of the one signaling frequency and relay 544 is energized only by the part of the current which is of the other signaling frequency. Furthermore, relays 534 and 544 are each adjusted to be operatively responsive to a current equal in value to somewhat more than half the maximum output of tube 525. In other words, the energy of signaling frequency Fl must exceed the sum of all other energy components to effect the operation of relay 534; and the energy of signaling frequency F2 must exceed the sum of all other energy components to effect the operation of relay 535; and, therefore, both of these relays cannot be operated at the same time. Relays 535, 531, 545, 541, 550, 510, 515, 580, 585, 590, 595, 596 and 599 are controlled by relays 534 and 544 to transmit direct current seizure, selective and supervisory signals to the two-way trunk circuit TWTl, outgoing trunk circuit OTl and incoming trunk circuit ITI. Relays 535 and 545 are slow-to-release relays which are directly controlled by relays 534 and 544. If relays 534 and 535 are operated responsive to current of signaling frequency Fl, relay 535 remains operated for a predetermined interval of time after relay 534 releases; and s gnaling current of frequency F2 must be received before relay 535 releases in order that a signal operation result from the successive operation of relays 534 and 544. Likewise if relays 544 and 545 are operated responsive to current of signaling frequency F2, relay 545 remains operated for a predetermined interval of time after relay 544 releases; and signaling current of frequency Fl must be received before relay 545 releases in order that a signal operation result from the successive operations of relays 544 and 534. These protections against false signal operation will be further explained in describing the operations involved on calls over toll line Tl2. Each of relays 534, 544, 531, 541, 550, 510, 505 and 590 is a polarized relay which has a normally energized biasing winding for normally maintaining the contacts in the position shown in the drawings. Relay 515 is a polarized relay having a normally energized operating winding which normally maintains the contacts in the position shown in the drawings and which has a normally energized biasing winding which effects the operation of the contacts to their alternate positions when the circuit through the operating winding is opened. The signal receiving circuit SR2 is similar to the circuit SRI the reference characters of like elements in these circuits having the same tens and units digits.

The above-mentioned network comprises condensers 505 and 500 and inductors 506 and 501 suppresses the outgoing signaling frequencies but transients produced at the beginning and the end of a signal impulse may be passed to the receiver and tend to cause false operation and interference with response to incoming signals. By using a band-pass filter tuned to the incoming signaling frequencies in place of the band suppression filter, the outgoing signaling currents could not cause false operation of the receiver. While the transients produced in an arrangement using a band-pas filter are not as high as those produced by the suppression network, they are of longer duration. Thus, in either case the transients may be high enough and long enough to cause interference unless the transients are reduced. Such a reduction may be effected by signal wave shaping or by adding a band-pass filter tuned to the outgoing signaling frequencies inserted between the secondary winding of transformer 429 and the back contacts of the cut-off relay 425 of the signal transmitter STI. If the filter between the directionally selective coil 420 and the signal receiver SRI is a band elimination filter, the signaling frequencies passed by the sending band-pass filter will be entirely rejected if the band width of the band-pass filter is equal to or less than the band width of the elimination filter. If the filter between the coil 420 and the signal receiver SRI is a band-pas filter, as above suggested, all frequencies within the pass band of the receiving filter are attenuated by the sending band-pass filter thereby preventing false operation and interference with the receiver.

Further explanation of the invention will be facilitated by describing in detail the circuit D- erations involved on a call over tool line TI2. Assume first that the plug P2 of a cord CD is inserted in jack J2 of the trunk circuit OTI in Fig. 1, thereby closing a circuit from battery in the operator's position circuit, through the sleeve conductor of the cord CD, as fully described in the aforementioned patent to King et al., through the sleeve conductor of jack J2, a back contact of relay I06 of trunk circuit OTI, and through the upper winding of relay I04 to ground. Relay I04 operates and closes a circuit for operating relay I33. Relay I 33 looks under control of relay I36 and closes a circuit for operating relay IIO. Relay IIO closes a circuit for lighting the busy lamp I2 associated with jack J2, closes circuits for operating relay I24, I36 and I51; closes a circuit including the inner lower contact of relay I52 for operatively energizing the upper winding of relay I40; closes a circuit through the middle biasing winding of relay I40 in series with ballast lamp I4I; connects ground to conductor I68 and disconnects the winding of relay I06 from conductor I68; disconnects conductor I10 from conductor I69 and the windings of relays 303 and 305 of the two-way trunk circuit TWTI; and connects conductor I10 to the winding of relay I05. The operation of relay I24 closes circuits for operating relays I I2 and I20, connects ground to conductor I68 and connects ground through its middle lower front contact, resistor I25, back contacts of relays II and I30, conductor I02, through the ring conductors of jack J 2, plug P2 and cord CD to operate a relay in the operators position circuit. The connection of ground to conductor I68 operates relay 304. Relay 304 looks to this ground and disconnects the windings of relay 305 and 303 from conductor I69. The operation of relay I36 opens the locking circuit of relay I33, but relay I33 holds under control of relay I04. Relay I33 is slow to release and relay I36 is slow to operate so as to prevent the momentary release of relay I40 if relay I40 operates before the plug P2 is fully seated in jack J2. The operation of relay I40 closes a circuit from battery through resistor I42, front contact of relay I40, conductor I64, through the windings of relays 43I, 433 and 435 of signal transmitting circuit STI to initiate the transmission of a seizure signal over toll line TI2. The operation of relay II2 opens the normaily closed circuit for operating pad control relay 4, closes a circuit for holding the busy lamp L2 lighted, and further opens the connection between conductors I69 and I10. The operation of relay I20 disconnects relay I34 and conductor I61 from the windings of retard coil I60 and conductors I6I and I62 and connects these conductors through the windings of relays I23 and I56 and conductor I63 to the winding of pad control relay 4. The operation of relay I36 opens a normally closed short circuit around the winding of relay I52 and closes a short circuit around resistor I53. Relay I52 i thereupon operated in a circuit through resistor I54, inner upper back contact of relay I30, conductor IOI, through the tip conductor of jack J2 and plug P2, thence to battery in the operators position circuit as described in the aforementioned King et al. patent. The operation of relay I52 closes a circuit for operating relay I5I and opens the operating circuit of relay I51. Relay I5I locks under control of relay I36 and connects the operating, upper winding of relay I40 through a back contact of relay I30, conductor I02, the ring conductors of jack J2, plug P2 and cord CD to battery in the operator's position circuit. When the dial key (not shown) is operated, the impulse contacts of the dial are connected in series with the operating winding of relay I40 as described in detail in the aforementioned King et al. patent; and, when the called toll route number is dialed, the impulse relay I40 operates and releases in response to each dial impulse to repeat the impulses to the signal transmitter STI as hereinafter described.

The aforementioned closing of the circuit through the front contact of relay I40 and windings of relays 43I, 433 and 435, in response to insertion of plug P2 in jack J 2, causes the operation of these relays. Relay 43I closes a circuit for operating the slow-to-release relay 432, and causes the release of the normally operated cutofl relay 425. The circuit in which relay 425 is normally operated is traced from ground at the back contact of relay 43I, through back contacts of relays 432 and 434, winding of relay 425, conductor 500, and through back contacts of relays 565 and 550, to battery. The release of relay 425 opens the connection between the talking conductors I6I and I62 of outgoing trunk circuit OTI and repeating coil 400, and connects the left windings of repeating coil 429 to th repeating coil 400. The operation of relay 433 closes a circuit from ground through the left contact of relay 445, right windings of repeating coil 429, back contact of relay 444, front contact of relay 433, conductor 450 and the right windings of the tone supply transformer 41I, to ground. The operation of relay 432 also connects conductor 450 to the back contact of relay 444, the front contacts of relays 432 and 433 being connected in parallel. The

alternating potential of frequency Fl generated by the oscillator tube 418 thereupon initiates signaling current of frequency Fl through transformer 4', and repeating coils 429 and 488, to transmit the first part of a seizure signal over toll line Tl2 to the signal receiver SR2 in the tandem oifice. The aforementioned operation of relay 435 closes a circuit for energizing the lower winding of relay 438 and opens the normally closed circuit through the lower winding of relay 44I. Upon energization of its lower winding, relay 438 opens its left contact and closes its right contact. When the circuit through the lower winding of relay I is opened by the operation of either or both of relays 435 and 438, condenser 439 is charged in series with resistor 448, the charging current being effective to hold relay 44I operated and thereby delay the operation of relay 444 for about .030 second after the operation of relay 433 initiates the signaling current of frequency Fl. When the charging current is no longer sufficient to maintain the operative energization of the lower winding of relay 44l, the normally energized upper winding of relay 44I effects the opening of the left contact and the closing of the right contact of this relay, thereby opening the energizing circuit through the upper winding of relay 445 and closing an operating circuit through the upper winding of relay 444. The operation of relay 444 disconnects the right windings of repeating coil 429 from conductor 458 and transformer 4H, and connects the right windings of repeating coil 429 through conductor 468 to the windings of tone supply transformer 48l. Since the closing of the operating circuit of relay 444 is delayed by the charging of condenser 439, the current of frequency Fl through coils 429 and 488 and toll line Tl2 continues for a minimum of .030 second before the operation of relay 444 terminates the signaling current of frequency Fl and initiates a signaling current of frequency F2. The upper winding of relay 445 continues to be energized, after the operation of relay I, by current charging condenser 442 in series with resistor 443. The opening of the left contact and closing of the right contact of relay 445 is thereby delayed for an interval of approximately .100 second, during which interval there is a current of frequency F2 through coils 429 and 480 and toll line TI2. The opening of the left contact of relay 445 ends the signaling current of frequency F2, and the closing of the right contact of relay 445 closes a circuit, including a back contact of relay 434, for energizing the winding of the cut-off relay 425. Reoperation of relay 425 disconnects repeating coil 429 from repeating coil 488 and reconnects coil 488 to conductors I6I and I62.

Thus a seizure signal, consisting of current of frequency of Fl for an interval of not less than .038 second followed immediately by current of frequency F2 for an interval of about .108 second, is transmitted over toll line Tl2 to the tandem office, through repeating coil 688, pads Plil and P62, the left windings of directionally selective coil 628, conductors IM and I82, condensers I83 and I84 and the right windings of transformer H8. The signal energy is applied through transformer M8 to the grid of amplifying tube H5, is amplified by tube "5, and is applied through transformer I28 to the grid of tube I25. The resistor IZI in the grid circuit of tube I25 effects a drop in potential which limits the amplitude of the output voltage. The network I38 offers substantially infinite impedance to current of frequency Fl, so that the signaling current of this frequency in the plate circuit of tube I25 is applied to and rectified by the full-wave rectifier I33; and the rectified current operatively energizes the upper winding of relay I34. When the signaling current of frequency Fl ceases, relay I34 releases. The network I48 offers substantially infinite impedance to current of frequency F2; and the signaling current of this frequency in the plate circuit of tube I25, immediately following the current of frequency F2, is applied to and rectified by the rectifier I43; and the rectified current operativeiy energizes the upper winding of relay I44. When the current of frequency F2 ceases, relay I44 releases. Since the current in the plate circuit of tube I25 is limited to a value which is suflicient for operating relay I34 only when substantially all of the current in line Tl2 is of frequency FI and which is sufficient for operating relay I44 only when substantially all of the current in line Ll is of frequency F2, the operation of either of these relays, when voice or other interfering currents are present, is prevented. The operation of relay I34 in response to the impulse of current of frequency Fl closes a circuit for operating relay I35 and closes a circuit through rectifier I38, the middle winding of relay I58, upper winding of relay I31 and resistor I38, to battery. Since ground is normally connected through back contacts of relays I31 and I45, to resistor I38, the operation of relay I34 is ineffective to energize the middle winding of relay I58 and upper winding of relay 131. When the current of frequency Fl ceases, relay I34 releases but relay I35 is a slow-to-releas relay and remains operated for approximately .020 second after the release of relay I34. The operation of relay I44, in response to signaling current of frequency F2 immediately following the current of frequency Fl, closes a circuit for operating relay I45 and closes a circuit through rectifier I46, the lower winding of relay I58, the upper winding of relay I41 and resistor I48. Since the operation of relay I44, in response to a seizure signal, occurs before relay I35 has released, the current in this circuit effects the operation of relays I41 and I58. The operation of relay I58 opens the normally closed energizing circuit of cut-off relay 625, this circuit being traced through back contacts of relays I58 and I85, conductor I88, winding of relay 625, and through back contacts of relays 634, 632 and 63I to ground. The operation of relay I41 prevents the connection of a short-circuiting ground to resistor I48 when relay I35 releases, and closes a circuit from ground through conductor I49, resistor 'I9I, lower winding of relay I98, and through resistor I93 to battery. Relay I98 operates, closing a circuit from ground through its left contact, winding of relay I95, conductor 9I8, lowermost back contact of relay 9l8 of outgoing trunk circuit 0T2, lower back contact of relay 9l2, conductor 969, back contact of relay 884 of two-way trunk circuit TWT2, back contact of relay 881, and through the winding of relay 883 to battery. Relays I95 and 883 are operated by the current in this circuit. The operation of relay I95 closes a circuit for operating relay I96 and connects ground to conductor I49 to maintain the energization of the lower winding of relay I98. Relay I96 also connects ground to conductor I49 and connects the right contact of relay I98 through resistor 191 to condenser I98 2,sss,21c

in preparation for receiving succeeding signals.

When the current of frequency F2 ends, relay 144 releases, causing the release of relays 145, 141 and 150, the release of relay 145 occurring about .020 second after the release of relay 144. The release of relay 150 closes the circuit for operating cut-off relay 625; and relay 625 closes the talking connection between toll line TI2 and conductors 96I and 962.

The aforementioned operation of relay 803, in response to the seizure signal, connects ground to conductor 823 to mark the two-way trunk circuit TWT2 and associated toll line TI2 busy in the banks of the toll route selectors. This ground potential is further connected through conductors 823 and 968, back contacts of relays 924 and 9I0 of the outgoing trunk circuit 0T2 to the winding of relay 906. Relay 985 operates, connects battery to the sleeve of jack J9 as a busy indication, and closes a circuit for operating relay 9I2. Relay 9I2 closes a circuit for lighting busy lamp L9 and opens the normally closed circuit for operating pad control relay 8i I. The aforementioned operation of relay 803 disconnects conductors 80I and 802 from conductors 82| and 822 leading to terminals in the banks of toll route selectors, and closes a circuit for operating relay 805 of trunk circuit TWT2 and line relay 851 of toll selector TS2. This circuit is traced from ground through the front contact of relay 190, winding of relay 195, conductor 910, back contacts or relays 9I0 and 9I2, conductor 969, back contact of relay 804, winding of relay 805, inner upper front contact of relay 803, conductor 961, thence in simplex through resistors MI and 922, inner back contacts of relay 920, windings of retard coil 960, outer back contacts of relay 930, conductors 8M and 802, outer front contacts of relay 803, conductors 85I and 852, back contacts of relay 860 of selector T52, resistors 86I and 862, to and through resistor 863 and another back contact of relay 860, through the winding of relay 851 to battery. The operation of relay 805 closes a circuit through resistor 806 for holding relay 803 and closes a circuit for operating relay 801. Relay 801 closes a holding circuit for relay 803 and connects ground to conductor 853 thereby operating relay 199 of signal receiver SR2. Relay 199 disconnects the winding of relay 180 from the contact of relay 510, to prevent the operation of relay 100 in response to signals incoming over line TI2 and connects relay 180 to conductor 649 for operation as hereinafter described during the transmission of signals over line TI2 to the calling ofiice. Relay 801 is a slow-to-release relay in order to maintain the connection of ground to conductor 853 during the response of relay 805 to dial impulses as hereinafter described. The operation of relay 851 closes a circuit for operating the slow-to-release relay 858. Relay 858 connects ground to conductor 853, and connects ground to conductor 859 leading to the alarm circuit which operates in usual manner to operate an alarm device if the selector T52 fails to complete its selective operations within a predetermined interval of time.

No further operations take place until the calling operator dials the desired toll route number, and also the called subscribers number if the called line is in an automatic ofiice. Relay I40 of outgoing trunk circuit OTI responds to the dial impulses, opening and closing the circuit through conductor I64 and the windings of relays 43l, 433 and 435 in response to each opening and closing of the impulse contacts of the dial. The release of relay 43I opens the operating circuit of relay 432 but relay 432 is slow in releasing and remains operated during the transmission of dial impulses. With relay 43I released and relay 432 operated, a circuit is closed in response to the first impulse of each train for operating relay 434; and since relay 434 is slow in releasing, it remains operated until relay 433 reoperates, after receiving the last impulse of the train. The operation of relay 434 causes the release of the cut-off relay 425 to disconnect coil 400 from conductors I6I and I62 during the transmission of dial impulses. The release of relay 435 opens the circuit through the lower winding of relay 438 and closes a circuit including the right contact of relay 438 for energizing the upper winding of relay 445. Relay 438 is held operated, after relay 435 releases, by current charging condenser 436. The energization of the upper winding of relay 445 effects the closing of its left contact; and, since relay 434 is operated and the right contact of relay 444 is closed, the right windings of coil 429 are connected through conductor 460 to transformer 48I to initiate a current of frequency F2 in line TI2. As soon as the current charging condenser 436 is no longer sufficient to maintain the closure of the right contact of relay 438 (about .030 second after re lay 435 releases), the lower winding of relay MI is again energized through the left contacts of relays 438 and 435. Relay 44I thereupon closes its left contact to maintain the energization of the upper winding of relay 445, the energization of this winding being maintained by current charging condenser 442 after relay 438 releases and before relay 4 operates. The opening of the right contact of relay 44I deenergizes the upper winding of relay 444; so that relay 444 opens its left contact to disconnect tone transformer 48I from coil 429 and closes its right contact to connect tone transformer 41I thereto, thereby ending the current of frequency F2 through coils 429 and 400 and toll line TI 2 and starting current of frequency FI. When relay 435 reoperates, at the end of the first dial impulse, it opens the circuit for energizing the lower winding of relay I and closes the circuit for energizing the lower winding of relay 438. Relay 438 immediately actuates its contacts but the actuation of the contacts of relay 4 is delayed while condenser 439 is charging. When relay 44I actuates its contacts, about .030 second after relay 435 reoperates, the upper winding of relay 444 is energized; and the coil 429 is disconnected from transformer 41! and again connected to transformer 48I, thereby ending the current of frequency FI and initiating current of frequency F2. Each release and reoperation of relay 435 in response to succeeding dial impulses in the same train effects the above-described cycle of operations whereby there is a current of frequency F2 followed by current of frequency FI through coils 429 and 400 and toll line TI2 for each dial impulse. At the end of the last impulse of the train, the current of frequency F2 continues for about .100 second while condenser 442 is being charged, whereupon the contacts of relay 445 are actuated to open the circuit through the right windings of coil 429. Relay 434 releases when relay 43I remains operated at the end of a train of impulses, thereby causing the reoperation of cut-off relay 425.

Dialing may start within .080 second after the plug P2 is inserted in Jack J2. In this case, relay 445 has not been operated to end the current of frequency F2 of the seizure signal, the release of relay 43l and operation of relay 433 maintain the deenergization of relay 425, and the release of relay 435 prevents the deenergizatlon of the upper winding of relay 445, for about .030 second while condenser 436 is charging and until relay 438 opens its right contact. Thus there is no interval between the current of frequency F2 of the seizure signal and the current of frequency F2 for the first dial impulse.

Each succeeding train of dial impulses effects the operation of the signal transmitter STI in like manner to transmit voice frequency impulses over toll line Tl2 to the receiver SR2 in the tandem office. The signal receiver SR2 responds to each train of dial impulses, the first train being effective to control the selective operation of selector TS2. The current of frequency F2, responsive to the first opening of the impulse contacts of the dial, causes the operation of relay 144. Relay 144 closes a circuit for operating relay 145 and closes a circuit through rectifier 146 and windings of relays 141 and 150. Since relay 135 is not then operated, the windings of relays 141 and 150 are short-circuited through back contacts of relays I35 and 141. When the current of frequency F2 through toll line Tl2 ceases and current of frequency Fl begins, relay 144 releases and relay 134 operates. Being slow in releasing, relay 145 remains operated for about .020 second after relay 144 releases; and the operation of relay 134, while relay 145 is operated, effects the operation of relays 135, 131 and 150. The operation of relay 150 causes the release of cut-oif relay 625 to disconnect coils 600 and 620 from talking conductors 86l and 962; and the operation of relay 131 prevents the short-circuiting of its upper winding, when relay 145 releases, and connects ground to conductor 139, thereby energizing the upper winding of relay 110. The energization of the upper winding of relay 110 effects the opening of its left contact and the closing of its right contact. The closing of the front contact of relay 110 is without effect, since relay 199 is operated; but the opening of the left contact of relay 110 opens the circuit through the upper winding of relay 115, the deenergization of this winding being delayed for about .030 second while condenser 'I1l is being charged in series with resistor I12. The deenergizatlon of the upper winding of relay 115 causes the closing of its left contact whereby ground is connected to conductor 194 to effect a reversal of the current in the lower winding of relay 190. Relay 190, thereupon, opens its left contact to cause the release of relays 195, 805 and 851. The closing of the right contact of relay 180 effects the discharge of condenser 198. When the current of frequency Fl ends, relays I34, 135 and 131 release, the release of relay 135 being delayed for .020 second. The release of relay 131 disconnects ground from conductor 139 but the upper winding of relay 110 remains energized for about .030 second while condenser 188 is being charged in series with resistor 191. When the charging current is no longer effective to operatively energize the upper winding of relay 110, the biasing winding effects the opening of the right contact and closing of the left contact of this relay. The upper winding of relay "5 is thereby energized to effect the opening of the left contact of this relay; and the resulting reversal of current through the lower winding of relay I90 causes the closing of the left contact of relay 180 to reoperate relays 195, 805

and 851. The current of frequency F2, immediately following the current of frequency Fl in line TI 2, causes the successive operations of relays 144 and 145. Since relay 135 has not yet released, the operation of relay 144 causes the operation of relay 141 and holds relay 150; and the operation of relay 141 connects ground to conductor 149. Since relay 196 is slow in releasing, it remains operated during the response of relays and to dial impulses and maintains the connection of ground to conductor 149. Each succeeding dial impulse in the train effects the reoperation of the relays of the receiver SR2 to effect a release and reoperation of relays I95, 805 and 851. When the current of frequency F2, following the last impulse of the train ceases, the release of relay 144 causes the release of relays 145 and 150. The release of relay 156 closes the circuit for operating cut-off relay 625 to reconnect coils 600 and 620 to talking conductor 96! and 962. Relay now maintains the connection of ground through the winding of relay to conductor 910 to hold relay 805 of the two-way trunk circuit TWT2 and to hold the line relay 851 of selector TS2 until an idle trunk is seized in the level corresponding to the first digit dialed by the calling operator.

Since relay 801 of two-way trunk circuit TWT2 and relay 858 of selector TSZ are slow in releasing, these relays remain operated during the re sponse of relays 865 and 851 to dial impulses. The release of relay 851 in response to the first impulse incoming over toll line Tl2 closes a circuit for operating the vertical stepping magnet 861 and the slow-to-release relay 869 in series. The operation of magnet 861 steps the brushes 81l, 812, 813 and 814 up the first level of the terminal bank. The vertical off-normal springs 864 are actuated when the shaft is moved out of normal position, closing a circuit including the front contact of relay 869 and conductor 853 for operating relay 865. Relay 865 locks, through the back contact of rotary stepping magnet 866 and a front contact of relay 865, to ground at a back contact of relay 860. When relay 851 reoperates at the end of the first impulse, magnet 861 releases but relay 869 is slow in releasing and remains operated until all of the impulses in the train created by the dialing of th first digit have been received by relay 851. Relay 858 is also slow in releasing and remains operated during the response of relay 851 to dial impulses. Each succeeding release and reoperation of relay 851 causes the operation and release of magnet 861, stepping the brushes of the selector up to the level corresponding to the digit dialed. When relay 869 releases, after all of the impulses in the train have been received, it closes a circuit for operating the rotary stepping magnet 866. This circuit is traced from battery through the winding of magnet 866, lower front contact of relay 865, back contact of relay 869, conductor 853, to ground at the upper front contact of relay 858. The operation of magnet 866 rotates the brushes one step, into engagement with the first set of terminals in the selected level. When magnet 866 opens its back contact, relay 865 releases. The release of relay 865 causes the release of magnet 866. If the terminal, with which test brush 813 is in engagement, is uarded by a ground potential, relay 865 reoperates when magnet 868 releases; the operating circuit being traced through the lower front contact of spring 864, back contact of magnet 866, lowermost back contact of relay 860 and test brush 813. The reoperation of relay 865 causes the operation of the rotary stepping magnet 866, therebyadvancing the brushes to the next set of terminals. The operation of magnet 866 releases relay 865; and the release of relay 865 releases magnet 866. Relay 865 and magnet 866 repeat this cycle of operations, to advance the brushes step by step, until brush 813 encounters a terminal not marked by ground potential; whereupon, relay 868 is operated to seize the idle set of terminals. The circuit for operating relay 868 is traced from battery through the winding of relay 865, front contact of the vertical off-normal springs 864, back contact of magnet 866, winding of relay 868, lower back contact of the 11th rotary step springs 856, through conductor 853 to ground at the upper front contact of relay 858. Relay 865 is marginal and does not reoperate in series with the winding of relay 868. Relay 868 disconnects the winding of relay 851 from conductors 85I and 852 and connects these conductors to brushes 8H and 812 thence to the trunk or selector circuit connected to the selected set of terminals. The release of relay 851 causes the release of relay 858 but ground is not thereby disconnected from conductor 853 since this conductor is connected to ground at the lower front contact of relay 881.

Assume first that the call is routed to an operator in the second toll office, that the terminals, in the level selected in response to the first digit dialed by the calling operator, are connected to incoming trunk circuits and that the terminals 816, connected to the trunk circuit 1T2, are selected by the trunk hunting operation of the selector TS2. In thi case, the operation of relay 860 closes a circuit for operating the line relay I885 of trunk circuit 1T2. This circuit is traced from battery through the winding of relay I805, thence in simplex through resistors I8I5, IOI6 and I8I1, the windings of retard coil I088, conductors I88I and I882, brushes 8H and 812, front contacts of relay 868, conductors 85I and 852, front contacts of relay 883, conductors MI and 882, back contacts of relay 938, winding of retard coil 968, back contact of relay 928, through resistors MI and 922, thence through conductor 961, inner upper front contact of relay 803, winding of relay 885, back contact of relay 804, conductor 969, lower back contact of relay 9 I 2, lowermost back contact of relay 9I8, conductor 918, winding of relay 195, to ground at the left contact of relay 198 of signal receiver SR2. Relays 195 and 885 are thereby held operated in series with relay I885. With relay I8I9 non-operated, resistor I8I8 is short-circuited; and, since the resistance of resistors IMS and NH together is equal to that of resistor [M6, the simplex is balanced so that pad control relay 934 of outgoing trunk circuit T2 remains non-operated. In the incoming trunk IT2 the winding of pad control relay I009 is bridged across the end of the simplex path; but this relay does not operat since the simplex circuit is balanced when an intertoll trunk, such as toll line TI2, is connected to the incoming trunk circuit 1T2. The aforementioned operation of relay I005 closes a circuit for lighting the answering lamp LIO and connects the source of ringing current I886 through a back contact of relay I83I and condenser I830 to talking conductor I042 to transmit a ringing tone to the calling operator to indicate that the toll operator in the called toll ofllce is being signaled. The calling operator having completed dialing restores the dialing key to normal, thereby reconnecting the operator's telephone through plug P2 and jack J2 to the outgoing trunk circuit OTI.

When the call is answered by insertion of the plug of a cord in jack J I8, relay I844 is operated in a circuit through conductor I043 and the sleeve conductor of jack J ID, to battery in the answering operators position circuit. Relay I844 closes a circuit for operating relay I834. Relay I834 opens the repeater termination, comprising resistor I033 and condenser I032 normally bridged across the talking conductors I84I and I842, and closes a circuit for operating relay I83I. This circuit is traced through the lower front contact of relay I834, front contact of relay I885, conductor I803, brush 813, lowermost front contact of relay 868, conductor 853, to ground at the lower front contact of relay 881. Relay I 83I locks under control of relay I808 through conductor I883, independent of relay I834. Relay I 03I extinguishes lamp LI8, opens the connection through ringing tone condenser I838 and closes a circuit for operating relay 8| I of two-way trunk circuit TWT2. This circuit is traced from ground, through the outer upper front contact of relay I834, back contact of relay I8I2, front contact of relay I83I, conductor I 884, brush 814, a front contact of relay 868, a back contact of the 11th rotary step springs 856 of selector TS2, and through conductor 854 to the winding of relay 8I I. Relay 8 closes a circuit from battery through resistor 942 of outgoing trunk circuit 0T2, conductor 966, front contact of relay 8, back contact of relay 8I8, conductor 965, back contact of relay 948, conductor 964 through the windings of relays 63I, 633 and 635, in series, to ground. Relay 63I closes a circuit for operating the slow-to-release relay 632 and causes the release of the normally operated cut-off relay 625. The release of relay 625 connects the left windings of repeating coil 629 to the repeating coil 688. The operation of relay 633 closes a circuit from ground through the right contact of relay 645, left windings of repeating coil 629, back contact of relay 644, front contact of relay 633, conductor 658 and the left windings of transformer 6" to ground. The operation of relay 632 also connects conductor 658 to the back contact of relay 644, the lower front contact of relay 632 and front contacts of relays 633 and 634 being connected in parallel. The alternating potential of frequency F3 generated by the oscillator 618 thereupon initiates signaling current of frequency F3 through transformer 6'II, repeating coils 629 and 688 to transmit the first part of an answering supervisory signal over toll line TI2 to signal receiver SRI. The operation of relay 635 closes a circuit for energizing the lower winding of relay 638 and opens a circuit including the right contact of relay 638 and the lower winding of relay 64I. Upon energization of its lower winding, relay 638 opens its right contact and closes its left contact. When the circuit through the lower winding of relay 64I is opened by the operation of either or both of relays 635 and 638, condenser 639 is charged in series with resistor 640, the charging current being effective to hold relay 64I operated and thereby delay the operation of relay 644 for about .038 second after the operation of relay 635 initiates the signaling current of frequency F3. When the charging current is no longer suiiicient to maintain the operative energization of the lower winding of relay 64I, the normally energized upper winding of relay 64I effects the opening of the right conof the operating circuit of relay 644 is delayed by' the charging of condenser 639, the current of frequency F3 through coils 629 and 600 and toll line TI2 continues for a minimum of .030 second before the operation of relay 644 terminates the signaling current of frequency F3 and initiates a signaling current of frequency F4. The upper winding of relay 645 continues to be energized, after the operation of relay 64I, by current charging condenser 642 in series with resistor 643. The opening of the right contact and closing of the left contact of relay 645 is thereby delayed for an interval of approximately .100 second, during which interval there is a current of frequency F4 through coils 629 and 600 and toll line TI2. The opening of the right contact of relay 645 ends the signaling current of frequency F4 and the closing of the left contact of relay 645 closes the circuit for energizing the winding of the cutoff relay 625. The reoperation of relay 625 disconnects repeating coil 629 from repeating coil 600 and reconnects coil 600 to talking conductors 96I and 962.

Thus an answering or off-hook signal, consisting of current of frequency F3 for an interval of about .030 second followed immediately by current of frequency F4 for an interval of about .100 second, is transmitted over toll line TI2 to the calling office, through repeating coil 400, pads PM and P42, the right windings of directionally selective coil 420, conductors 50I and 502, condensers 503 and 504 and the left windings of transformer 5I0. The signal energy is applied through transformer 5I0 to the grid of amplifier tube 5I5, is amplified by tube 5I5, and is applied through transformer 520 to the grid of tube 525. The resistor 52I in the grid circuit of tube 525 effects a drop in potential which limits the amplitude of the output voltage. The network 530 offers substantially infinite impedance to current of frequency F3 so that the signaling current of this frequency in the plate circuit of tube 525 is applied to and rectified by the full wave rectifier 533; and the rectified current operatively energizes the upper winding of relay 534. The network 540 offers substantially infinite impedance to current of frequency F4; and the current of this frequency in the plate circuit of tube 525, immediately following the current of frequency F3, is applied to and rectified by the rectifier 543; and the rectified current operatively energizes the upper winding of relay 544. When the current of frequency F4 ceases, relay 544 releases. Since the current in the plate circuit of tube 525 is limited to a value which is sufficient for operating relay 534 only when substantially all of the current incoming in line TI2 is of frequency F3 and which is sufiicient for operating relay 544 only when substantially all of the current in line TI2 is of frequency F4, the operation of either of these relays, when voice or other interfering currents are present, is prevented. The operation of relay 534, in response to current of frequency F3, closes a circuit for operating relay 535 and closes a circuit through rectifier 536, the middle winding of relay 550, upper winding of relay 531 and resistor 538 to battery. Since ground is normally connected through back contacts of relays 531 and 545 to resistor 538, the operation of relay 534 is ineffective to energize the middle winding of relay 550 and upper winding of relay 531. When the current of frequency F3 ceases, relay 534 releases but relay 535 is a slow-to-release relay and remains operated for approximately .020 second after the release of relay 534. The operation of relay 544, in response to signaling current of frequency F4 immediately following the current of frequency F3, closes a circuit for operating relay 545 and closes a circuit through rectifier 546, the lower winding of relay 550, the upper winding of relay 54'! and resistor 548. Since the operation of relay 544, in response to the answering signal, occurs before relay 535 has released, the current in this circuit effects the operation of relays 54'! and 550. The operation of relay 550 causes the release of the cut-off relay 425. Relay 5" prevents the connection of a short-circuiting ground to resistor 546 when relay 535 releases. and closes a circuit from ground through conductor 549, resistor 59I, lower winding of relay 590 and through resistor 593 to battery. Relay 590 operates closing a circuit from ground through its right contact, winding of relay 595, conductor I10, a front contact of relay H0, and through the winding of relay I05. Relays 595 and I05 are operated by the current in this circuit. The operation of relay 595 closes a circuit for operating relay 596 and connects ground to conductor 549 to maintain the energization of the lower winding of relay 590. Relay 596 also connects ground to conductor 549 and connects the left contact of relay 590 through resistor 591 to condenser 598 in preparation for receiving succeeding signals. When the current of frequency F4 ends, relay 544 releases causing the release of relays 545, 541 and 550, the release of relay 545 occurring about .020 second after the release of relay 544. The release of relay 550 closes the circuit for operating cut-off relay 425 to close the connection between coil 400 and conductors I6I and I62. The operation of relay I05 opens the circuit through the lower winding of relay I04 thereby leaving the high resistance upper winding of relay I04 in series with the sleeve conductor of jack J2 and plug P2 so as to extinguish the supervisory lamp SL2 as an answering or off-hook signal.

While the answering operators telephone set is connected to trunk IT2, battery is connected through jack J I0 to both of conductors WM and I042, thence through the windings of retard coil IOI0, through resistor I0 and relay IOI2, conductor IOI3, through the upper back contact of relay I035 and winding of relay I02I to ground. Relay I02I operates closing a circuit for operating relay IOI9. Relay IOI9 disconnects pad control relay I009 and opens the short circuit around resistor IOI8 and short-circuits resistor I III I to unbalance the simplex circuit through conductors IO0I and I002 so as to effect the operation of pad control relay 934 of outgoin trunk circuit 0T2. Relay 934 closes a circuit through conductor 963 for operating relay 6I I. Relay 6 opens the circuit for operating relay 6I0; and with relay 6I0 released, pads PSI and P62 are included in the talking circuit. After the operators telephone set is disconnected from trunk IT2 and this trunk is connected through the cord to another trunk or toll line, relay I02I is held operated if this other trunk or toll line is a switching trunk or tributary line but is released if it is atoll line or intertoll trunk. The release of relay I 02I causes the successive release of relays IOI9, 934 and 6H and the operation of relay 6I0 to short-circuit the pads PSI and P62. If the intertoll trunk associated with the outgoing trunk circuit T2 is one which is not provided with transmission pads, relay 934 is not provided. If the intertoll trunk associated with the outgoing trunk circuit 0T2 is one which does not require the short-circuiting of the pads of a line or trunk to which the incoming trunk circuit IT2 is connected by the answering operator's cord, resistors MI and 922 of the outgoing trunk circuit 0T2 would be of such value as to unbalance the simplex circuit and thereby cause the operation of relay I009 of the incoming trunk circuit IT2. The operation of relay I009 closes a circuit for operating relay I035. Relay I035 disconnects relay I02I from conductors I04I and I042 and connects battery through resistors I031 and I038 to conductors I04I and I042 to hold operated the pad control relay of the trunk or line to which the trunk circuit IT2 is connected by the answering operators cord.

The talking connection between outgoing trunk circuit OTI and incoming trunk circuit IT2 includes conductors IM and I02, front contacts of relay I30, conductors I 6| and I62, front contacts of relay 425, resistors 42I and 422, repeating coil 400, toll line TI2, repeating coil 600, resistors 62I and 622, front contacts of relay 625, conductors 96I and 962, back contacts of relay 930, conductors 80I and 802, front contacts of relay 803, conductors 85I and 652, front contacts of relay 860, brushes 8H and 812, conductors IO0I and I002, condensers I050 and conductors I04I and I042.

If the answering operator disconnects from jack J I0, relay I044 releases causing the release of relay I034 to disconnect ground from conductor I004 and effect the release of relay 8| I. The release of relay 8 causes the release of relays 63 I, 633 and 635 of signal transmitter ST2, thereby eflecting the transmission of an on-hook signal consisting of current of frequency F4 followed by current of frequency F3. The release of relay 63I closes a circuit through a front contact of relay 632 and the front contact of relay 199 for operating relay 180. The operation of relay 180 causes the operative energization of the upper winding of relay 185 for about .550 second while condenser 182 is discharging, the reason for this being hereafter described in the case of release of the connection while a line busy signal is being transmitted to the calling operator on a call to a subscriber in a local dial office. The release of relay 63I also closes a circuit for operating the slow-to-release relay 634. Being slow in releasing relay 632 remains operated long enough, after relay 63I releases. to cause the operation of relay 634. The operation of relay 634 causes the release of cut-off relay 625 to disconnect line TI2 from conductors 96I and 962 during the transmission of the "onliook signal. The release of relay 635 opens the circuit through the lower winding of relay 638 and closes a circuit including the left contact of relay 638 for energizing the upper winding of relay 645. Relay 638 is held operated for about .030 second after relay 635 releases, by current charging condenser 636. The energization of the upper winding of relay 645 effects the closing of its right contact; and, since relay 634 is operated and the right contact of relay 644 is closed, the left windings of coil 629 are connected through conductor 660 to transformer 68I to initiate current of frequency F4 in line TI2. When the current charging condenser 636 i no longer effective to maintain the operative energization of the lower winding of relay 638, the circuit through the upper winding of relay 645 is opened at the left contact of relay 638; and the lower winding of relay 64I is again operatively energized in the circuit including the right contacts of relays 638 and 635, the contacts of relay 64I being thereby restored to normal. After relay 638 releases and until relay 64I closes its right contact, the energization of the upper winding of relay 645 is maintained by current charging condenser 642. When relay 64I reoperates, the opening of the left contact of this relay deenergizes the upper winding of relay 644; and the closing of the right contact of relay 64I maintains the energization of the upper winding of relay 645. Relay 644 opens its right contact, thereby disconnecting tone transformer 68I from coil 629; and closes its left contact, thereby con necting tone transformer 61I to coil 629. Thus the impulse of current of frequency F4 is ended and an impulse of current of freuency F3 is started through coils 629 and 600 and toll line TI2. When relay 632 releases, it causes the release of relay 634, the current of frequency F3 being maintained for about .750 second before the release of relay 634 disconnects coil 629 from conductor 650 and transformer EN. The release of relay 634 causes the release of relay and the release of relay 180 causes the operative energization of the upper winding of relay while condenser 'I8I is discharging. As long as either of relays 634 or 185 is operated, the operating circuit for relay 625 is open and the coil 600 disconnected from talking conductors 96I and 962.

In response to the on-hook" signal transmitted over line TI2 to the signal receiver SRI, relay 544 is operated by the current of frequency F4 and relay 534 is operated by the current of frequency F3. Relay 544 closes a circuit for operating relay 545 which remains operated until relay 534 closes circuits for operating relays 535, 531 and 550. The operation of relay 550 causes the release of cut-off relay 425; and the operation of relay 531 connects ground to conductor 539 to energize the upper winding of relay 510. Relay 510 opens the normally closed circuit through the upper winding of relay 515 and closes a circuit for operating relay 580. The operation of relay 580 closes a circuit for charging condenser 58I and closes a circuit for discharging the normally charged condenser 582 through the winding of relay 585; whereby relay 585 is 0peratively energized for about .550 second. About .020 second after relay 510 operates, relay 515 closes its right contact; whereby the current through the lower winding of relay 590 is reversed and relay 590 releases, opening its right contact and closing its left contact. The opening of the right contact of relay 590 causes the release of relays 595 and I05. Since relay 534 is held operated for about .750 second, relay 510 is operated and relay 515 released long enough for relay 596 to be released. The release of relay I05 connects ground to the lower winding of relay I04, to decrease the resistance in the circuit through conductor I03 to cause the lighting of the supervisory lamp SL2 associated with plug P2 of cord CD in the manner described in the aforementioned King et al. patent. The calling operator is thus given an on-hook signal. When relay 534 releases at the end of the current of frequency F3, relays 535, 531 and 550 release. The release of relay 531 release relay 

